# Amphimerycidae

Amphimerycidae is an extinct family of artiodactyls that was endemic to western Europe that lived from the Middle Eocene to the Early Oligocene. With a taxonomic history extending as far back as 1804, the family was formally recognized by the Swiss palaeontologist Hans Georg Stehlin in 1910 and contains two genera: Amphimeryx and Pseudamphimeryx. Both amphimerycid genera are very similar to each other in terms of skull and dental anatomy but do have specific differences from each other. Both genera are best known from their fused cuboid bone and navicular bone, which together make up a single "cubonavicular bone" of the hind legs. This trait had long been used in support of the idea that they were ruminants by taxonomists. However, their classification to the Ruminantia had also been rejected by other taxonomists later on due to differences in dentition; the systematic position of the Amphimerycidae and close relatives in relation to the wider Artiodactyla (or Cetartiodactyla), as a result, is unclear.
The Amphimerycidae lived in western Europe, which at the time was an archipelago that was isolated from the rest of Eurasia. This meant that it lived in a tropical-subtropical environment with various other faunas that also evolved with strong levels of endemism. Pseudamphimeryx was the first-occurring genus but then was replaced by Amphimeryx in the Late Eocene. Amphimeryx lived up to the Early Oligocene, having apparently briefly survived after the Grande Coupure turnover event.

## Taxonomy

The earliest history of the Amphimerycidae was in 1804 when the French naturalist Georges Cuvier erected Anoplotherium minimum in 1804, stating that unlike with other species assigned to Anoplotherium (A. commune, A. medium, and A. minus), A. minimum lacked known postcranial fossil evidence. In 1822, he emended A. minimum to A. murinum (noting that the species still lacked postcranial evidence unlike with other Anoplotherium species) and classified it to the subgenus Dichobune. In 1848, the French palaeontologist Auguste Pomel erected the genus Amphimeryx for the reclassified species A. murinus, arguing that it was close to ruminants in affinity.
In a palaeontology textbook dating back to 1891–1893, the German palaeontologist Karl Alfred von Zittel classified Amphimeryx to the artiodactyl family Xiphodontidae. Swiss palaeontologist Hans Georg Stehlin reclassified it and the newly recognized Pseudamphimeryx to their own family, the Amphimerycidae, in 1910. He also noted that while Amphimeryx was long thought thought to have been closely related to Xiphodon, the possibility that both the Amphimerycidae and Xiphodon independently acquired similar anatomical traits cannot be eliminated. The French palaeontologist Jean Viret gave a formal diagnosis of the Amphimerycidae in 1961.

### Classification

For much of the taxonomy history of the amphimerycids, their placements within or outside the Ruminantia had been disputed and still remains so today. In 1941, the American palaeontologist Edwin H. Colbert wrote about evolutionary affinities of fossil and extant ruminants, comparing Archaeomeryx to other artiodactyl genera like Amphimeryx, Hypertragulus, Gelocus, and Tragulus. He said that fossil evidence of Amphimeryx was not complete during his time of study but suggested that it may have been a primitive member of clade Tragulina but that Archaeomeryx was overall more primitive than it. He then classified Amphimeryx to its own ruminant superfamily Amphimerycoidea, separating it from other traguline superfamilies like the Hypertraguloidea and Traguloidea. His classification was followed by another American palaeontologist George Gaylord Simpson in 1945.
In 1961, Viret reclassified the Amphimerycidae and the Xiphodontidae into the artiodactyl clade Ancodonta, therefore removing the former from the Ruminantia. Similarly, American palaeontologists S. David Webb and Beryl E. Taylor in 1980 argued that the Amphimerycidae had historically been tied to the Ruminantia due to postcranial convergences but otherwise had more in common with xiphodonts than ruminants in terms of dentition. However, they chose to tentatively reclassify the Xiphodontidae and Amphimerycidae to the Tylopoda instead, although they did also suggest the possibility of them being a sister group to ruminants. On the other hand, in 1997, the American palaeontologists Malcolm McKenna and Susan K. Bell reclassified the Amphimerycidae into the Ruminantia.
According to Jörg Erfurt and Grégoire Métais in 2007, the similarities of amphimerycids with ruminants are thought to have been an instance of parallel evolution, in which amphimerycids and ruminants independently gained similar traits. While amphimerycids have typically been excluded from the Ruminantia due to dental characteristics, it does not eliminate the possibility of them being sister taxa to ruminants by the latter independently gaining longer legs and more selenodont (crescent-shaped) dentition. Its affinities, along with those of other endemic European artiodactyls, are unclear; the Amphimerycidae, Anoplotheriidae, Xiphodontidae, Mixtotheriidae, and Cainotheriidae have been determined to be closer to either tylopods (i.e. camelids and merycoidodonts) or ruminants. Different phylogenetic analyses have produced different results for the "derived" selenodont Eocene European artiodactyl families, making it uncertain whether they were closer to the Tylopoda or Ruminantia.
In an article published in 2019, Romain Weppe et al. conducted a phylogenetic analysis on the Cainotherioidea within the Artiodactyla based on mandibular and dental characteristics, specifically in terms of relationships with artiodactyls of the Palaeogene. The results retrieved that the superfamily was closely related to the Mixtotheriidae and Anoplotheriidae. They determined that the Cainotheriidae, Robiacinidae, Anoplotheriidae, and Mixtotheriidae formed a clade that was the sister group to the Ruminantia while Tylopoda, along with the Amphimerycidae and Xiphodontidae split earlier in the tree. The phylogenetic tree used for the journal and another published work about the cainotherioids is outlined below:
In 2020, Vincent Luccisano et al. created a phylogenetic tree of the basal artiodactyls, a majority endemic to western Europe, from the Palaeogene. In one clade, the "bunoselenodont endemic European" Mixtotheriidae, Anoplotheriidae, Xiphodontidae, Amphimerycidae, Cainotheriidae, and Robiacinidae are grouped together with the Ruminantia. The phylogenetic tree as produced by the authors is shown below:
In 2022, Weppe conducted a phylogenetic analysis in his academic thesis regarding Palaeogene artiodactyl lineages, focusing most specifically on the endemic European families. One large monophyletic set consisted of the Hyperdichobuninae, Amphimerycidae, Xiphodontidae, and Cainotherioidea based on dental synapomorphies, of which the hyperdichobunines are paraphyletic in relation to the other clades. In terms of the amphimerycids, while the clade consisting of P. renevieri and A. murinus was recovered as a sister group to the other endemic artiodactyl clades, the placement of P. schlosseri has rendered the Amphimerycidae paraphyletic in relation to the derived amphimerycid species and other families. He argued that the Amphimerycidae thus needs a systemic revision for which P. schlosseri would be assigned to a new genus and removed from the Amphimerycidae.

## Description

The Amphimerycidae is a family of small-sized artiodactyls whose species ranged in weight from 0.4 kg (0.88 lb) to 1.5 kg (3.3 lb) total. Amphimerycids are defined in part as having an elongated snout and large orbits that are widened in their backs. The dental formula of the Amphimerycidae is 3.1.4.33.1.4.3 for a total of 44 teeth, consistent with the primitive dental formula for early-middle Palaeogene placental mammals. The incisors (I/i) are shovel-shaped, have sharp edges on their crowns, and have horizontal positions in relation to the dental row. The canines (C/c) are incisiform (incisor form) and therefore differ little with the incisors themselves. The premolars (P/p) are elongated and may generally be separated by diastemata (gaps between teeth). The lower premolars have three lobes, or developed areas on their crowns. The upper molars (M/m) are more developed in form and are generally subtriangular in shape, although some may be more rectangular. They have five crescent-shaped (selenodont) tubercles and sometimes a partial hypocone cusp that may be present in all species. Amphimerycids differ from ruminants, particularly the basal clade Tragulina, in the retentions of their first premolars and their high levels of specialization in their selenodonty and number of cusps in their molars. Their dentitions more closely resemble those of xiphodonts or dacrytheriines than of ruminants. The overall selenodonty and brachyodonty (low-crowned teeth) of amphimerycids suggest that they were adapted towards folivorous (leaf-eating) dietary habits. Pseudamphimeryx and Amphimeryx, both known by multiple skull specimens, have very similar forms but differ based on a few characteristics. Amphimeryx is distinguished from Pseudamphimeryx in part by the more well-developed occipital crest present on the snout of the latter. While the peak of the skull's top of Amphimeryx slopes down to its front area, that of Pseudamphimeryx appears initially concave at the occipital crest's front, ascends slightly, and then finally slopes down.
Both amphimerycid genera have especially prominent occipital and sagittal crests, the latter of which divides into two less prominent branches behind the fronto-parietal suture that extend up to the supraorbital foramen. The frontal bones of both amphimerycid genera are large plus flat, being particularly sizeable in their supraorbital portions; this trait is more pronounced in Amphimeryx. The lacrimal bone of both amphimerycids, but especially in Amphimeryx, has an extensive pars facialis and is quadrangular in shape, narrowing at its front. The orbit is large, is positioned back in relation to the overall skull, is wide at its back area, and is more curved at its upper compared to lower edge. There is no difference between both amphimerycids in terms of the orbits, suggesting based on their morphologies that the snouts of both genera are elongated. The optic foramen, located in the sphenoid bone, extends more forward in Amphimeryx than in Pseudamphimeryx. While the nasal bone is not as well-preserved in Amphimeryx fossils, the frontonasal suture is implied to have formed a W shape on the skull's upper surface like that of Pseudamphimeryx. Both amphimerycid genera also have similar, although not identical, medial positions of the infraorbital foramen in the maxilla. The palatine bones of Amphimeryx and Pseudamphimeryx are narrower at their front than back ends.
Both amphimerycid genera are best known by the "cubonavicular" bone (fused cuboid bone and navicular bone of the hind legs) recorded in multiple species; the morphology of the astragalus of P. renevieri further attests to anatomical support of the fused bone. This trait has also been recorded in ruminants, suggesting that the amphimerycids and ruminants independently acquired the trait in an instance of parallel evolution. The primitive state of the astragalus sets Amphimeryx apart from ruminants; the approximately equal sizes of its trochleas and more rounded edge of its sustentacular facet also sets the genus apart from the Cainotheriidae. In Amphimeryx, the metatarsal digits III and IV are elongated and partially fused to each other while the side digits II and V are greatly reduced to small but needlelike forms. Digit III measures 50 mm (2.0 in) long while digit II is no more than 14 mm (0.55 in) long. These traits are similarly recorded in derived ruminants, which have tetradactyl (four-toed) feet, absent digit I, reduced digits II and V, and fused digits III and IV that make up the cannon bone (the now-extinct primitive ruminants had pentadactyl (five-toed) feet, unreduced digits II and V, and unfused digits III and IV). Like other artiodactyls with only two elongated digits in each foot (digits III and IV), Amphimeryx was functionally didactyl, meaning that it walked only on its two elongated toes per foot.

## Palaeoecology
### Middle Eocene

For much of the Eocene, a hothouse climate climate with humid, tropical environments with consistently high precipitations prevailed. Modern mammalian orders including the Perissodactyla, Artiodactyla, and Primates (or the suborder Euprimates) appeared already by the Early Eocene, diversifying rapidly and developing dentitions specialized for folivory. The omnivorous forms mostly either switched to folivorous diets or went extinct by the Middle Eocene (47–37 Ma) along with the archaic "condylarths". By the Late Eocene (approx. 37–33 mya), most of the ungulate form dentitions shifted from bunodont (or rounded) cusps to cutting ridges (i.e. lophs) for folivorous diets.
Land connections between western Europe and North America were interrupted around 53 Ma. From the Early Eocene up until the Grande Coupure extinction event (56–33.9 mya), western Eurasia was separated into three landmasses: western Europe (an archipelago), Balkanatolia (in-between the Paratethys Sea of the north and the Neotethys Ocean of the south), and eastern Eurasia. The Holarctic mammalian faunas of western Europe were therefore mostly isolated from other landmasses including Greenland, Africa, and eastern Eurasia, allowing for endemism to develop. Therefore, the European mammals of the Late Eocene (MP17–MP20 of the Mammal Palaeogene zones) were mostly descendants of endemic Middle Eocene groups.
The Amphimerycidae, and by extent the first genus Pseudamphimeryx, is first recorded by the appearance of P. schlosseri in the Swiss locality of Egerkingen α + β, dating back to MP14. Both families would have coexisted with perissodactyls (Palaeotheriidae, Lophiodontidae, and Hyrachyidae), non-endemic artiodactyls (Dichobunidae and Tapirulidae), endemic European artiodactyls (Choeropotamidae, Cebochoeridae, and Anoplotheriidae), and primates (Adapidae). The stratigraphic ranges of the early species of Amphimeryx also overlapped with metatherians (Herpetotheriidae), cimolestans (Pantolestidae, Paroxyclaenidae), rodents (Ischyromyidae, Theridomyoidea, Gliridae), eulipotyphlans, bats, apatotherians, carnivoraformes (Miacidae), and hyaenodonts (Hyainailourinae, Proviverrinae). Other MP13-MP14 sites have also yielded fossils of turtles and crocodylomorphs, and MP13 sites are stratigraphically the latest to have yielded remains of the bird clades Gastornithidae and Palaeognathae.
The unit MP16 records the appearances of P. renevieri and P. pavloviae, both of which are recorded from the MP16 French locality of Robiac. Other mammal genera that cooccur in the site include the herpetotheriids Amphiperatherium and Peratherium, apatemyid Heterohyus, nyctitheriid Saturninia, rodents (Glamys, Elfomys, Plesiarctomys, Ailuravus, Remys), omomyids Pseudoloris and Necrolemur, adapid Adapis, hyaenodonts Paroxyaena and Cynohyaenodon, carnivoraformes Paramiacis and Quercygale, palaeotheres (Propalaeotherium, Anchilophus, Plagiolophus, Pachynolophus, Palaeotherium), lophiodont Lophiodon, hyrachyid Chasmotherium, cebochoerids Acotherulum and Cebochoerus, choeropotamid Choeropotamus, tapirulid Tapirulus, anoplotheriids (Dacrytherium, Catodontherium, Robiatherium), robiacinid Robiacina, and xiphodonts (Xiphodon, Dichodon, Haplomeryx).
After MP16, a faunal turnover occurred, marking the disappearances of the lophiodonts and European hyrachyids as well as the extinctions of all European crocodylomorphs except for the alligatoroid Diplocynodon. The causes of the faunal turnover have been attributed to a shift from humid and highly tropical environments to drier and more temperate forests with open areas and more abrasive vegetation. The surviving herbivorous faunas shifted their dentitions and dietary strategies accordingly to adapt to abrasive and seasonal vegetation. However, the environments were still subhumid and covered by subtropical evergreen forests. The Palaeotheriidae was the sole remaining European perissodactyl group, and frugivorous-folivorous or purely folivorous artiodactyls became the dominant group in western Europe.

### Late Eocene and Early Oligocene

The Late Eocene unit MP17 records as many as four total species of Pseudamphimeryx: P. renevieri, P. havloviae, P. salesmei, and P. hantonensis. MP17a confirms the continued occurrence of P. renevieri in the French locality of Fons 4; MP17b is the latest unit that Pseudamphimeryx occurs and records both P. renevieri and P. pavloviae from another French locality of Perrière. Starting at MP18, Amphimeryx makes its first appearance and therefore succeeds Pseudamphimeryx. The MP18 locality of La Débruge of France indicates that A. murinus coexisted with a wide variety of mammals, namely the herpetotheriid Peratherium, rodents (Blainvillimys, Theridomys, Plesiarctomys, Glamys), hyaenodonts (Hyaenodon and Pterodon), amphicyonid Cynodictis, palaeotheres (Plagiolophus, Anchilophus, Palaeotherium), dichobunid Dichobune, choeropotamid Choeropotamus, cebochoerids Cebochoerus and Acotherulum, anoplotheriids (Anoplotherium, Diplobune, Dacrytherium), tapirulid Tapirulus, xiphodonts Xiphodon and Dichodon, cainothere Oxacron, and the anthracothere Elomeryx.
MP20 marks the last known appearance of A. murinus, but the species A. riparius is apparently recorded solely from the MP21 French locality of Ronzon. Many other artiodactyl genera from western Europe disappeared as a result of the Grande Coupure extinction event, the Ronzon locality indicates that the Amphimerycidae may have survived past the event but went extinct not long after. The causes of the extinctions of many other mammals in western Europe have been attributed to negative interactions with immigrant faunas (competition, predations), environmental changes from cooling climates, or some combination of the two.